1. Field of the Invention
The present invention relates to an aftertreatment apparatus and method for treating diesel exhaust in a motor vehicle.
2. Description of the Prior Art
Diesel engines are efficient, durable and economical. Diesel exhaust; however, can harm both the environment and people. To reduce this harm governments, such as the United States and the European Union, have proposed stricter diesel exhaust emission regulations. These environmental regulations require diesel engines to meet the same pollution emission standards as gasoline engines.
One part of diesel exhaust includes diesel particulate material. Diesel particulate material is mainly carbon particles or soot. One way to remove soot from diesel exhaust is with diesel traps. The most widely used diesel trap is a diesel particulate filter which nearly completely filters the soot without hindering exhaust flow. As a layer of soot collects on the surfaces of the inlet channels of the filter, the lower permeability of the soot layer causes a pressure drop in the filter and a gradual rise in the back pressure of the filter against the engine. This phenomenon causes the engine to work harder, thus decreasing engine operating efficiency. Eventually, the pressure drop in the filter and decreased engine efficiency becomes unacceptable, and the filter must either be replaced or the accumulated diesel soot must be cleaned out.
The filter is cleaned of accumulated diesel soot by burning-off or oxidation of the diesel soot to carbon dioxide which is known as regeneration. Regeneration of an existing filter is superior to filter replacement, because no interruption for service is necessary.
The regeneration process is either passive or active. Passive regeneration occurs when the filter becomes so filled with carbon particles that heat increases within the exhaust system due to excessive back pressure. The increased heat raises the temperature of the carbon to a point where the carbon ignites. This design, however, often results in thermal shock or melt down of the filter, high fuel penalty and poor filtering action.
Active regeneration uses heat generated by an outside source under controlled conditions to initiate combustion of the diesel soot. Soot slowly burns for a brief period. During this burn, the temperature in the filter rises from about 400°-600° C. to about 800°-1000° C. The highest temperatures occur near the exit end of the filter due to the cumulative effects of the wave of soot combustion from the entrance face to the exit face of the filter as the exhaust flow carries the combustion heat down the filter. Electrical power, fuel burners and microwave energy are all used as outside heat sources.
Under certain circumstances, a so-called “uncontrolled regeneration” occurs when the onset of combustion coincides with, or is immediately followed by, high oxygen content and low flow rates in the exhaust gas (such as engine idling conditions or low loads). During an uncontrolled regeneration, the combustion of the soot may produce temperature spikes within the filter which can thermally shock and crack, or even melt, the filter.
In addition to capturing carbon soot, the filter also traps ash particles, such as metal oxides, that are carried by the exhaust gas. These particles are not combustible and, therefore, are not removed during regeneration. If temperatures during uncontrolled regenerations are sufficiently high, however, the ash may eventually sinter to the filter or react with the filter, thus resulting in partial melting.
Furthermore under light loads and idling conditions, the stream of exhaust particles is too dilute to efficiently burn the soot to regenerate the filter. Under such light loads, the stream of exhaust particles is at a lower pressure and temperature than the exhaust stream at medium to full loads. This can inhibit the regeneration of the filter or lead to uncontrolled regeneration of the filter.
Therefore, it would be advantageous to increase the efficiency of the regeneration of the filter while inhibiting uncontrolled regenerations. It would be further advantageous to increase the efficiency of the diesel aftertreatment without using costly parts or requiring bulky additional equipment.